1. Field of the Invention
The present invention generally relates to electrical power distribution systems. In particular, the present invention relates to a link disconnect box that allows for the easy isolation of a network protector switch from load side connections.
2. Description of the Related Art
The distribution of electricity in urban centers involves medium voltage (up to 35 kV) electrical cables (called “feeders”) connecting a power substation to the utility customer base. In the vicinity of the customer, connections are made from the feeder to a transformer that reduces the voltage to one capable of being utilized by the customer. In order to sectionalize the feeder, that is, to isolate the feeder in the event of an electrical fault, three phase power breakers called network protectors are installed on the low voltage side of the transformer. The customer connection side (“load side”) of various network protectors can be shorted together to create a low voltage electrical grid (“network”), which provides reliable power service to the utility customer. Secondary power lines extend from the low voltage electrical grid to the consumers, and as a result a failure of any single transformer in the grid does not disrupt power to the consumers.
There are presently tens of thousands of these network protectors in operation in urban electrical centers throughout the world. Each network protector operates as an automatic switch, connecting or disconnecting the related transformer to the network based upon the monitoring of electrical conditions. A network protector comprises a large three phase switch/breaker element installed in an enclosure, which can be a submersible tank or a non-submersible cabinet. The switch element is connected on one side to the low voltage side of a transformer and on the other side to the network. Because the load side of the network protector can be connected to the load side of other network protectors in a network system, there is an enormous amount of electrical energy available at any one network protector. Protecting utility workers from this energy while they are servicing network protectors is of paramount importance.
As indicated above, the network protector acts as a three-phase switch between the low-voltage side of a transformer and the customer. When a transformer, network protector or both need to be serviced, the feeder to the transformer can be isolated so as to prevent high voltage current from reaching the transformer using an electrical switch between the feeder and the transformer. However, because of the networked connections on the low voltage side of the network protector, it is not possible to isolate the network protector itself from low voltage current. Consequently, the customer side of the network protector remains energized, and this can present serious safety issues for linemen, particularly in the form of arc flash when servicing the network protector.
It is therefore desirable to provide a link disconnect box that can be retrofitted onto the existing terminals of a network protector so that, once installed, linemen can readily disconnect a network protector from a low voltage network.